You don't want trashmail, or anyone else for that matter, censoring the CONTENT of your messages. So use reply masking - but also remember to use a VPN/proxy/Tor or some such to hide your IP. Plus other tools to hide your replying email client fingerprint (if your paranoid). Or send all your replies directly from the trashmail site as sent emails rather than replying from your email client (this option sends no identifying data).